The ancient Egyptians it appears were the first culture in history
to construct a solar calendar. It contained twelve months with each month
having thirty days. There were an additional five days, which were often
referred to as the epagomenal days at the end of the year that made 365
days. The months were Thoth, Phaophi, Adyr, Choiak, Tybi, Mecheit, Phamenoth,
Pharmuti, Pachon, Payni, Epiphi, and Mesore. The five epagomenal days were
held to be the birthdays of the deities Osiris,
Aroueris, Set,
Isis,
and Nephthys
respectively.

In one Egyptian myth, the sky-goddess Nut,
wife of Ra,
loved the earth-god Geb and became pregnant by him. Upon his discovery of
the pregnancy Ra cursed Nut and ordained that she would not give birth on
any day in any month of the year. Thoth,
the god of wisdom, also was a lover of Nut, gambled with the goddess of
the moon winning from her one seventeenth of each day. Together this number
composed five entire days, which Thoth attached to the year. Now these days
belonged to no month of the year so on them Nut bore the children of Geb.
A.G.H.

Source:

Greer, John Michael. The New Encyclopedia of the Occult.
St. Paul. MN. Llewellyn Worldwide. 2005. pp. 145-146